The present invention relates to electronically steered antennas, and more particularly to an electronically steered, phased array antenna having a plurality of independent antenna modules, wherein each of the antenna modules are rotated relative to an adjacent module by a predetermined number of degrees to significantly improve cross polarization isolation of the antenna.
An electronically steered, phased array antenna uses a large quantity of independent antenna modules laid out in a flat grid pattern. Each of these modules has a pair of conductive elements xe2x80x9cAxe2x80x9d opposed by 90xc2x0 to each other. A highly simplified illustration of one such antenna module is shown in FIG. 1. Specific details of the construction of such antennas are disclosed in U.S. Pat. No. 5,276,455 to Fitzsimmons et al., issued Jan. 4, 1994 and hereby incorporated by reference into the present application. In a perfect antenna, these conductive elements would be isolated from each other. Since they are not, a portion of the signal intended for one of these conductive elements ends up being received (or transmitted) by the other element. When all of the module element signals are summed together, the desired signal can end up being corrupted by the non-isolated signal. An increase in the amount of corruption causes a decrease, or deteriorization, in cross polarization isolation.
The preferred solution to improving cross polarization isolation in the summed element antenna pattern is to design and build a module with perfectly isolated elements. This is often impossible and/or expensive. A second proposed solution is to isolate the summed signal from one polarization, phase shift it and subtract a certain amount of it from the summed signal of the other polarization. The practical implementation of this approach, however, has not been achievable to date.
A typical electronically steered antenna has a thin layer of low dielectric material adhered to the top of the antenna aperture. Its purpose is to improve the impedance match at the input of each antenna module over the scan angle. This layer is called the wide angle impedance match (i.e., WAIM). The expected cross polarization isolation of an antenna with a WAIM is shown in FIG. 2. This expected cross polarization isolation example is taken from an antenna with a module grid pattern as shown in FIG. 3. The WAIM is only able to maximize the cross polarization isolation at a particular module spacing, in this case at 30xc2x0, 90xc2x0, 150xc2x0, 210xc2x0, 270xc2x0 and 330xc2x0 azimuth scan angles, which corresponds to the shortest distance possible between adjacent modules. With perfectly isolated elements within each module, the overall cross polarization isolation should look like that provided in FIG. 2, with high numbers (best case cross polarization isolation) at azimuth scan angles of 30xc2x0, 90xc2x0, 150xc2x0, 210xc2x0, 270xc2x0 and 330xc2x0 and lower numbers (worst case cross polarization isolation) at azimuth scan angles of 0xc2x0, 60xc2x0, 120xc2x0, 180xc2x0, 240xc2x0 and 300xc2x0. It will be appreciated that the xe2x88x9230xc2x0 line in FIG. 3 is equal to 330xc2x0 in FIG. 2. It will also be appreciated that the WAIM improves the overall cross polarization isolation.
With imperfectly isolated elements within each module, the cross polarization isolation appears as presented in FIG. 4. Referring to FIG. 4, the cross polarization isolation pattern no longer repeats every 60xc2x0 corresponding to the distance between modules at a given azimuth angle, but is now resolved into a pattern where the best case cross polarization isolation is about equal to the expected worst case cross polarization isolation. Looking at azimuth scan angle 0xc2x0-90xc2x0, the best cross polarization isolation lies between 0xc2x0 and 60xc2x0 and falls off to the worst case cross polarization isolation at 90xc2x0 (13 dB). The pattern that improves and worsens until the second worst case cross polarization isolation occurs at approximately 170xc2x0 of azimuth scan angle (15.2 dB). This pattern then repeats from 180xc2x0 to 360xc2x0.
To explain the change from a pattern that varies every 60xc2x0 to one that is considerably worse and varies roughly every 180xc2x0, it is instructive to look at the element orientation within the antenna. Currently, all of the module elements used in present day phased array antennas are typically aligned with one another as shown in FIG. 5. A simulation of this orientation in an array of 1528 modules has been done by The Boeing Co. and cross polarization isolation was determined for four azimuth scan angles (0xc2x0, 30xc2x0, 60xc2x0 and 90xc2x0) at an elevation scan angle of 60xc2x0. This information is presented in Table 1 below:
These four data points of Table 1 compare favorably with the measured data as shown in FIG. 4, thus verifying the simulation approach.
Simulation of an individual module shows that the amount of energy reflected (i.e., return loss) off of each radiating element when compared to its neighboring element also demonstrates a 180xc2x0 pattern. This implies that the total summation of individual module element outputs is masking the expected cross polarization isolation pattern. This simulation is illustrated in Table II below.
Accordingly, it is a principal object of the present invention to provide an electronically scanned, phased array antenna which provides significantly improved cross polarization isolation over prior developed phased array antennas. More particularly, it is an object to provide a phased array antenna having a plurality of antenna modules which are arranged in such a pattern that the overall cross polarization isolation of the antenna is significantly improved.
The above and other objects are provided by an electronically scanned, phased array antenna in accordance with a preferred embodiment of the present invention. The antenna of the present invention includes a plurality of independent antenna modules which are spaced in a grid arrangement. Each antenna module has a pair of radiating elements which are offset relative to each other by a predetermined angle. In one preferred form, this angle comprises 90xc2x0. The antenna modules are arranged such that they form a plurality of columns and rows. Each column has adjacent antenna modules rotated by approximately 90xc2x0 from one another. Accordingly, no two adjacent antenna modules in each column are aligned in identical orientations.
In one preferred form, the antenna modules in each column are rotated such that the modules are arranged in a repeating pattern of 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0. In this manner, the worst case cross polarization isolation performance of each module is not summed together at the same angle as every other module, but rather is broken up over the entire azimuth scan angle.
The phased array antenna of the present invention thus does not require significant manufacturing modifications nor any added expense to improve the cross polarization isolation of the antenna. By simply rotating adjacent antenna modules disposed in each column of the grid of modules, the present invention avoids the problem of summing the worst case cross polarization isolation performance of each module at a given azimuth scan angle.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.